This invention pertains generally to the art of combustion turbines and in particular to that portion of the art relating to airfoil stator vane structures.
As is well known to those skilled in this art, turbine stator vanes in a combustion turbine work in a very severe environment. The temperatures of the hot combustion gas leaving the combustor baskets in their annular array are not uniform when the gas reaches the first stage stator vanes. Large temperature variations exist in the circumferential direction, as well as in the radial direction. With the typical current design of multiple airfoil vane segments, non-uniform heating of the different airfoils leads to premature creep-fatigue failures of the segments.
Vane segments are produced by the investment casting process. With a multiple airfoil casting, it is difficult to achieve a uniform and controlled solidification of the molten metal. Areas containing porosity and macrosegregation are commonly found in the airfoil-to-shroud intersection vicinity of the multiple airfoil segments. Of couse, these casting defects lower the material low-cycle fatigue and creep properties.
The lowered material properties combined with high thermal strains at the shroud-to-airfoil intersection vicinity have caused premature failures in multiple airfoil segments. The high thermal strains are caused by nonuniform heating and cooling of the redundant multiple airfoil structure. Thus, obviously, the life of the vane segment casting can be increased if the thermal strains are reduced and if the material properties are improved.
It is the aim of this invention to provide an improved structure which reduces structural constraints and discontinuities and in addition improves the mechanical properties of the investment casting in high stress areas.
While my invention proceeds from the basis that the casting will be a single vane segment, as distinct from multiple vanes in a segment, it is acknowledged that the bare concept of the use of single vane segments is disclosed in U.S. Pat. No. 3,689,174. However, my invention includes an arrangement which is structurally different in a number of its aspects and correspondingly is considered to provide advantages relative to an arrangement as shown in the noted patent.